LeetCode刷题笔记（链表）：linked-list-cycle-ii

骑猪看日落 2022-06-01 00:39 123阅读 0赞

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*  **转载请注明作者和出处：[http://blog.csdn.net/u011475210][http_blog.csdn.net_u011475210]**
 *  **代码地址：[https://github.com/WordZzzz/Note/tree/master/LeetCode][https_github.com_WordZzzz_Note_tree_master_LeetCode]**
 *  **刷题平台：[https://www.nowcoder.com/ta/leetcode][https_www.nowcoder.com_ta_leetcode]**
 *  **题  库：Leetcode经典编程题**
 *  **编  者：WordZzzz**

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*   *  题目描述
     *  解题思路
     *  C版代码实现

## 题目描述 ##

Given a linked list, return the node where the cycle begins. If there is no cycle, returnnull.

Follow up:  
Can you solve it without using extra space?

## 解题思路 ##

*  同linked-list-cycle-i一题，使用快慢指针方法，判定是否存在环，并记录两指针相遇位置(Z)；
 *  有环的情况下， 求链表的入环节点：遍历链表，把每个元素指向下个链表的指针赋值为NULL，则循环要么在链表结尾停止，要么在环状链表入口处停止。

## C++版代码实现 ##

/**
     * Definition for singly-linked list.
     * struct ListNode {
     *     int val;
     *     ListNode *next;
     *     ListNode(int x) : val(x), next(NULL) {}
     * };
     */
    class Solution {
    public:
        //判断是否为环状链表
        bool hasCycle(ListNode *head) {
            if(head == NULL || head->next == NULL)
                return false;
            ListNode *fast = head;
            ListNode *slow = head;
            while(fast->next != NULL && fast->next->next != NULL){
                slow = slow->next;
                fast = fast->next->next;
                if(slow->val == fast->val)
                    return true;
            }
            return false;
        }
    
        //检测函数
        ListNode *detectCycle(ListNode *head) {
            if(hasCycle(head)) {
                ListNode *temp = NULL;
                while(head ->next) {
                    temp = head ->next;
                    head ->next = NULL;
                    head = temp;
                }
                return head;
            }
            else
                return NULL;
        }
    };

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[http_blog.csdn.net_u011475210]: http://blog.csdn.net/u011475210
[https_github.com_WordZzzz_Note_tree_master_LeetCode]: https://github.com/WordZzzz/Note/tree/master/LeetCode
[https_www.nowcoder.com_ta_leetcode]: https://www.nowcoder.com/ta/leetcode